How safe are you when flying abroad? It is a complex question with no easy answers.

Security procedures for your transatlantic flight on a U.S. airline can begin the moment you phone to book your ticket.

At the outset, the reservation clerk may ask -- persistently -- for your passport number, as happened when I reserved a seat earlier this year on a Pan Am flight to Frankfurt. I assumed Pan Am simply wanted to verify that I had a valid passport. In fact, this is one way in which Pan Am -- and other U.S. airlines -- screen their passenger lists in advance for potential terrorists.

You will encounter yet another -- and more obvious -- step in the screening process on the day of departure. As you approach the check-in counter, a uniformed security agent will stop you to ask a few curious but pointed questions. Did you pack your suitcases yourself? Have they been out of your sight since you left home? Are you carrying parcels out of the country for anyone? The queries are intended, in part, to make sure you are not an unsuspecting dupe for a terrorist who has planted a bomb in your bags. Young travelers, especially, are subject to such questioning.

But the questioning can get more intense if the security agent has any cause for suspicion. An American writer headed for France last winter was halted and interviewed at length at the Chicago airport. He is a frequent traveler, and his most recent passport had been issued by a U.S. embassy office outside the United States -- a circumstance unusual enough to raise doubts. The agent demanded a full explanation. In such instances, screeners are attempting to assure that you are who you say you are before you are allowed to board the plane.

Of course, your carry-on luggage will be X-rayed for hidden weapons, and you will be required to step through a metal detector. These are standard security procedures implemented almost two decades ago at airports around the world as a way of thwarting in-flight hijackings. The irony is that conventional weapons have become difficult to hide, so terrorists are manufacturing small plastic bombs -- such as the one that destroyed Pan Am Flight 103 over Lockerbie, Scotland, in 1988 -- that elude standard metal detectors.

Once you have landed in Europe, you almost certainly will come under continued scrutiny -- at least until you are outside the airport. In Vienna, for example, police guards in groups of three and four patrol the airport carrying machine guns, often accompanied by decidedly unfriendly-looking attack dogs. Their main goal is to prevent terrorist attacks on travelers inside the terminal.

These are only the most visible in a wide array of official security measures -- both old and new -- devised to protect air travelers in an unsafe world plagued by fanatical terrorists capable of mass murder to achieve political ends.

It may come as a surprise to many travelers that U.S. and foreign carriers flying the Atlantic do not operate under the same security demands. The Federal Aviation Administration requires U.S. airlines to take specific steps to screen passengers and luggage on flights to Europe and the Middle East -- considered the areas where the potential for a terrorist attack is greatest. Foreign carriers flying to or from the United States are under no obligation to adhere to the same strict procedures.

Pan Am has made a competitive issue of the dual standard, contending that U.S. carriers are safer because foreign airlines "are not required to meet the rigorous FAA standards the U.S. carriers have implemented."

"Major foreign airlines do not put their passengers through the extensive screening and check-in procedures that U.S. airlines now do," said Pan Am Chairman Thomas Plaskett in a full-page ad that appeared in The Washington Post and three other newspapers last month. "Nor do they X-ray all baggage on every international flight (even baggage in transit) as Pan Am does."

Some security experts contend, however, that foreign carriers don't need the extra security because the terrorist threat they face is much less. Among them, Billie Vincent, former director of security for the Federal Aviation Administration, chooses to fly on foreign rather than U.S. airlines to Europe and the Middle East. He does so, he says, because U.S. security measures are "not sufficient" to counter the terrorist threat. Vincent, who left the FAA in 1986, is an airport security consultant in Arlington.

"We feel that we are as safe as any of the European carriers, if not safer," responds American Airlines spokesman John Hotard. "U.S. carriers easily are as safe as foreign carriers," says FAA spokesman Fred Farrar.

The question of who is right in this debate (if anyone) seems unresolvable, given the intangible nature of the terrorist threat and the secrecy surrounding many of the security procedures adopted in the United States and abroad. A look at the issues involved, however, may help individual travelers draw their own conclusions.

The Lockerbie disaster, for all of its tragic consequences, has resulted in substantially improved security for the air traveler. This is one of the conclusions reached by the President's Commission on Aviation Security and Terrorism. "The level of security awareness in the international aviation community has increased dramatically," says the commission's report on the bombing of Pan Am 103, issued May 15. "Many governments have taken steps to improve air carrier and airport security."

As part of its findings, however, the 182-page report faults both Pan Am and the FAA for security flaws prior to Lockerbie. But it says they have subsequently corrected the deficiencies -- although it took Pan Am 10 months to clear up its problems after the crash. In its full-page ad, published the day the report was issued, Pan Am says it is adding 500 more security personnel, "not only meeting new, more demanding FAA requirements, but surpassing them."

One of the FAA's failures, the commission has concluded, is that the agency "is far too reactive to problems instead of anticipating them." Bruce Hoffman, a security expert for the Rand Corp. in Santa Monica, agrees. "The big challenge is to stay one step ahead of terrorists," he says. "There has to be a constant commitment.

"After terrorists found it difficult to hijack planes, they didn't give up," he says. "They found another way." By this he means aircraft bombings. What's next in the escalation of terrorist warfare if terrorism is not eliminated?

Hoffman's look into the future is frightening. Should security efforts -- including the installation of expensive new bomb-detecting machines in airports -- make bombings impractical, terrorists may manage to acquire surface-to-air missiles to shoot down aircraft from the sky. Some 40 countries have missiles in their weapons stockpiles, Hoffman says.

These and other issues are part of an international debate over what constitutes an adequate security system for air travel. Among the major considerations, most of which were raised by the Commission on Aviation Security and Terrorism:

Passenger convenience. Some airlines fear that U.S. safety procedures could become too onerous, causing passengers to switch to foreign airlines operating under less stringent requirements. "It's definitely a problem," says Philip Davidoff, president-elect of the American Society of Travel Agents in Alexandria.

Because security procedures for U.S. airlines are more time-consuming, he says, "the business traveler is asking to fly on foreign carriers because they don't want to put up with U.S. procedures. It's blatantly unfair." When passengers are being screened, long lines can build in front of the check-in counters of U.S. airlines; this very often is not the case with foreign airlines. At Washington-Dulles International Airport, for example, Lufthansa does not interview passengers at check-in.

About 45 percent of U.S. international passengers fly on foreign carriers, according to the commission's report.

The commission studied airports in Britain, France and West Germany. Among the findings:

"West German officials, along with those of other European countries, oppose the extensive U.S. questioning process, which causes long lines and congestion in their airports. They believe it virtually impossible to isolate the suicide bomber or the innocent dupe who unknowingly carries a bomb aboard a plane."

Who pays? The cost of providing security is astronomical. Currently, U.S. airlines -- some of which are in shaky financial condition -- must pay to hire the large security staffs required to screen passengers and luggage on international flights. Pan Am says it is spending $63 million in 1990 on security activities. Foreign governments generally pick up at least part of the cost of security measures for their airlines, according to the commission report.

One looming expense for U.S. carriers is the cost of installing thermal neutron analysis machines, a device designed to detect plastic bombs. The FAA has decided the airlines should share the $150 million cost of 150 of the machines, to be placed in 40 airports in the United States and at U.S. airline facilities at airports abroad.

The airlines contend the government should be responsible for funding such extraordinary security costs. It is government policy, they argue, that is the real target of terrorists, and that the U.S. airlines are only symbols of that target.

Complicating the debate on this issue is the commission's recommendation that the FAA defer its decision requiring U.S. airlines to buy the machines. The commission expresses doubts about their effectiveness and says their huge cost "will inevitably stifle interest in developing new and superior technologies." One possibility is strengthened aircraft interiors that might isolate the effects of an in-flight explosion.

The FAA in fact did purchase the first six of the thermal neutron machines, and one already is in operation at the TWA terminal at John F. Kennedy International in New York, according to FAA spokesman Farrar. One also is being installed at the Pan Am terminal in Miami, as well as at the United Airlines terminal at Washington-Dulles and at Gatwick airport in London for use by U.S. carriers. A decision where to place the other two has yet to be made.

Training for security workers. "You're only as good as the people who work for you," says Hoffman of the Rand Corp. Realizing this, U.S. airlines have taken steps recently to increase the professionalism of their security staffs. These are the people who monitor the airport X-ray machines and screen passengers checking in for an international flight. Some carriers, such as American Airlines, employ their own security force; others, among them Pan Am, hire security firms on contract.

Last fall, the Air Transport Association of Washington, which represents the major U.S. carriers, formulated a list of standards for security workers. The airlines are currently writing the standards into contracts, according to Tim Neale, a spokesman for the association. The FAA expects to adopt them shortly.

The standards require that personnel involved in passenger or baggage screening must be U.S. citizens with a high school diploma. They must be at least 18 years old and be able to communicate effectively in English. They must undergo background, drug and alcohol checks. And they must score positively on an aptitude test designed to show whether they would perform satisfactorily in the job.

In addition, successful applicants must complete a 12-hour training course developed by the Air Transport Association. A textbook and videotapes have been produced to accompany the lessons.

Is this sufficient? Some security experts are doubtful, arguing that security agents for El Al, the Israeli airline, undergo weeks of training, not just a couple of days. Most passenger screening techniques used by U.S. airlines have been adapted from El Al, generally regarded as the airline with the most intensive security.

The nature of the threat. Though the Lockerbie disaster is most prominent in the public mind, U.S. airlines have not been the sole target of terrorists. Of the 13 explosions aboard commercial aircraft since 1986, says Hoffman, only three have involved U.S. carriers.

The two most recent airplane bombings, both last year, involved UTA, a French airline flying from Brazzaville, Congo, and Avianca, a Colombian airline en route from Bogota to Cali. They represent the terrorist threat to international travelers in less industrialized countries. The Colombian crash, says the commission report, "may also signify the entry of an additional terrorist threat, on this side of the Atlantic, from the drug cartels."

The world's major airports have substantially improved their defenses since Lockerbie, but this is not true in many countries that do not have the money to spend on security. "Many of these airports," says the commission report, "have no perimeter fencing, no security for airplanes and no screening procedures for passengers." The United States and some European countries are providing security training and other assistance to many of these airports as a safeguard to the traveling public.

Travelers abroad may encounter strict security measures at airports where terrorist attacks on the terminal building itself are considered a possibility. Armed guards patrol many terminals, and at a few you have to present a ticket and passport before you are admitted inside. Frequent travelers report that some airports X-ray all luggage for concealed weapons before you can step inside the terminal.

For understandable reasons, the FAA and the world's airlines tend to keep many of their security procedures secret, although others obviously are visible. Are they sufficient, if implemented conscientiously and efficiently?

"It's a matter of faith," says Christopher Witkowski, director of the Aviation Consumer Action Project in Washington. "We don't know what the security programs entail." His organization was founded by Ralph Nader in 1971 to work, in part, for improved airline safety and security.

Witkowski thinks airline passengers should have a voice in the development of security standards. Most decisions are reached in discussions between airline representatives and the FAA. But passengers and airline crew personnel, he says, could offer a viewpoint "not influenced by money or the bureaucracy." He suggests adding public representatives to the Aviation Security Advisory Committee, which was created last year to advise the FAA.

How does the U.S. government go about protecting U.S. travelers flying abroad?

The measures in place vary depending on the perceived danger of the threat. At airports in Europe and the Middle East, the FAA requires U.S. carriers to use extra security precautions because the threat is deemed high. These "extraordinary procedures" were strengthened on an emergency basis nine days after the Lockerbie disaster, according to the commission report.

Security measures, standard and extraordinary, include:

Airport inspections. Following the hijacking of TWA Flight 847 out of Athens in 1985, the United States instituted a program of inspecting foreign international airports used by U.S. airlines. Also included are airports used by foreign carriers as their last stop before flying to this country. A total of 247 foreign airports in 99 countries are assessed, according to the commission report.

The airports must be inspected annually, usually by a two-member team making a three- to five-day visit. The inspectors don't actually test the effectiveness of security procedures, says the report, but they do note what measures are in place. If an airport proves unsatisfactory, it has 90 days to correct the deficiency. If the corrections aren't made, the State Department is required to issue a travel advisory, which is distributed to the traveling public. This has happened only once, in a situation involving the Manila airport in 1986.

Baggage examination. Checked baggage on U.S. carriers is X-rayed on all flights within Europe and the Middle East or on flights originating in Europe and the Middle East that are headed for the United States, according to the FAA's Farrar. In addition, some passengers are requested to open their bags so that they can be physically inspected, and the empty suitcases are X-rayed.

Baggage-passenger reconciliation. U.S. airlines must make sure that every passenger who checks a piece of luggage is on the plane before it takes off. If the passenger fails to board, the luggage must be removed. "Passenger/baggage reconciliation is the bedrock of any heightened civil air security system," says the commission report.

The fatal Pan Am flight in 1988 may have carried an extra bag that did not belong to a passenger on the flight, according to the report. The bag probably came from a connecting flight on another airline.

Pan Am's baggage reconciliation procedures now have an assist from computer technology, says Jeff Kriendler, vice president for corporate communications. When you check your luggage, the number on your baggage tag is entered into the computer with your name. Also, your name is on the tag attached to the suitcase. If you don't board, the airline knows which suitcase is yours. When the suitcase is loaded into a container, a part of the baggage tag is removed and pasted on the outside of the container. Should the airline have to search for your suitcase, it can quickly trace it to the right container.

My Pan Am flight from London to Washington this winter was delayed for perhaps half an hour while security personnel searched for a missing family. They apparently had misunderstood the departure time and were found in the duty-free shop. Had they not appeared, presumably their luggage would have been removed before we took off.

Baggage reconciliation delays can extend from an hour to an hour and a half, says Hotard of American.

In many foreign airports, all checked luggage is lined up outside an aircraft. Before you board, you identify yours. Those pieces not claimed are left behind.

How safe are you when flying abroad? Very safe, probably -- but only if government and airline security personnel here and abroad are doing their job properly.

One thing is certain, however. Because of the safety measures adopted both in this country and abroad, the chance of becoming the victim of a terrorist is very slim. "So many flights take off every day," says Doris Davidoff, co-owner of Belair Travel in Bowie. "You've got much greater odds losing your life on the Beltway."

Is there anything travelers can do as individuals to assure a safe flight? Security experts suggest that you:

Become aware of the political climate at destinations where they are headed. You don't want to be a victim in a dispute between factions within a region or country.

Fly an airline that is economically sound and has a reputation for providing its passengers with good service. A solid airline should provide you with solid security.

Copies of the "Report of the President's Commission on Aviation Security and Terrorism" can be purchased for $13 from Government Printing Office bookstores. The main bookstore is at 710 North Capitol St. There is a branch at 1717 H St. NW. Copies also may be ordered by phone: 783-3238.